Method and apparatus for the production of alkali cellulose



Jan. I, 1946. w. R. SCHMITZ. JR 2,392,259

METHOD AND APPARATUS FCR PRODUCTION OF ALKALI CELLULOSE Filed Sept. 10, 1943 Ric/lard SC/Lffli{;,J/: INVENTOR.

. v A TTORNE Y Patented Jan. 1, 1946 UNITED STATES PATENT orrlcl-z METHOD AND APPARATUS FOR THE PRO- DUCTION F ALKALI CELLULOSE William mama Schmitz, Jr., Wilmington, net,

assignor to E. I. du Pont de Nemours & Company, Wilmington, Del, a corporation of Dela- Application September 10, 1943, Serial No. 501,754 Claims. (01. 260-233) This invention relates to the preparation of alkali cellulose. More particularly, it relates to an eflicient and easily controlled process for the preparation, in a continuous manner, of an alkali cellulose of uniform and desirable composition, which alkali cellulose is eminently suited for conversion into a cellulose derivativ such as sodium cellulose xanthate for use in the viscose process of producing filaments and yarns.

Alkali cellulose for use in the spinning of yarns by the viscose process should possess a. uniform composition throughout and should be in a form that is capable of reacting rapidlyand uniformly with carbon bisulfide to form sodium cellulose xanthate. It must, of course, be of the proper composition (usually expressed as "press weight According to the other basic steeping procedure, the cellulose (solid material) does not retain its original pressed physical form, but is transformed together with an excess amount of being used in either a batch or a continuous ratio," which term indicates the ratio of weight of the final mass of alkali cellulose to the weight of the original unreacted air-dry cellulose forming that mass). The press weight ratio should not exceed 3:1 and should preferably be about Two basically different procedures have been developed for the steeping of solid materials in an excess of liquid media. of them, the solid material is retained in its original physical form during the steeping operation and during the removal of the excess steep ing liquor. This procedure, which has found almost universal use in the commercial preparation of alkali cellulose for use in viscose yarn spinning processes, requires that the original cellulosic materialbe of a proper physical form either as cut sheets of definite size, strength and surface characteristics, or as a continuous web supplied in roll form. If supplied in the form of cut sheets, the material is steeped in an aqueous solution of sodium hydroxide and pressedin an intermittent or batch manner in the steeping presses customarily employed in the viscose process. If supplied in roll form, the steeping and pressing operation can be performed on the cellulose web in a continuous manner. Such a process is disclosed in U. S. Patent No. 2,145,862

to Collings et. al. Although the viscose art has generally adopted the batch form of the above procedure (utilizing cellulose in cut sheet form) it has fully appreciated the serious difiiculties presented by it with regard to obtaining the desired uniform penetration of the causticsoda solution to all parts of the cellulose material. Various changes have been made in the apparatus and technique employed'but these have served only to alleviate and not to eliminate the difilculties.

According to one manner, the process has not generally been adopted by the viscose art. This isprimarily because of the difliculty in removing the excess caustic from th slurry to yield an alkalicellulose of the proper press weight ratio. It has been proposed to effect this removal of caustic from the slurry in a batch manner wherein the slurry is placed in a rotatable, porous container, the excess caustic being removed through the side walls of the container by the centrifugal force developed during the rotation. However, it is diificult to control in this manner the amount of caustic solution removed so that the press weight ratio of the alkali cellulose produced is not always uniform.

80 ess.

A device has recently been proposed (Schmitz, Patent No. 2,308,031) to effect the separation of a slurry into its solid and liquid components in a continuous manner, the device being admirably suited for use with a slurry comprising a fibrous material such as cellulose suspended in a liquid medium such as caustic soda solution. As described in that specification, the device comprises a tank provided in its lower portion with an entry pipe and at its upper portion with two co-acting cylindrical rolls mounted in a parallel opposed manner so as to form a bite between them. These rolls are adapted to be rotated in a manner such that the roll surfaces at the bite move in an upward direction. The rolls are hollow and have of about -20 lbs. per sq. in. The interior of the rolls through their connection with the exterior of the tank is maintained at atmospheric pressure and the pressure differential between the interior of the tank and the interior of the rolls forces the slurry to the roll surfaces. There, the separation of the slurry is effected, the liquid component passing through the roll surfaces to the interiors and to waste while the fibrous solid component is formed on the exterior of the rolls into mats of increasing thickness. As the rolls rotate, these mats are brought into the bite of the rolls and are there subjected to an increased pressure which removes still further liquid from them and forms them into a solid slab of fibrous material.

While this device will operate to satisfactorily separate a slurry of alkali cellulose and caustic solution into its component parts, its operation and the exact nature of the slab of solid material produced by it are dependent on such factors as the chemical and physical characteristics of the entering slurry; the pressure differential existing between the interior of the tank and the interior of the rolls (when the interior of the rolls is at atmospheric pressure, this is equal to the gauge pressure within the tank) the relative size of the rolls and the tank: the size and number of perforations on the roll surfaces: the rotational speed of the rolls; and the width of the bite between the rolls.

All of the above factors can be readily controlled except the chemical and physical characteristics of the slurry entering the tank to be pressed by the rolls. The cellulose used in making the slurry often varies considerably in the degree to which it will swell, and consequently the degree to which the resultant alkali cellulose will swell. There may also be variations in moisture content of the cellulose, or there may' be. at times, slight fluctuations in the feeding rate of the cellulose to the slurry producing apparatus. Such unavoidable variables often result in variations in the chemical and physical characteristics of the slurry.

The temperature of the slurry has an important effect on the physical and chemical characteristics of the slurry. This is because of the effect of temperature on the degree of swelling of the alkali cellulose and its attendant effect on the ease of separating the slurry into its component parts. The dimculty of removing excess caustic from the alkali cellulose is directly proportional to the degree of swelling of the alkali cellulose.

Obviously, all of these above several factors must be taken into account and certain operating conditions satisfied if a satisfactory process capable of acting in a continuous manner for the preparation of a uniform and desirable alkali cellulose product is to be obtained. The above described device in operation effects a reduction in volume of the slurry, the reduction being achieved through the removal of the liquid medium from the slurry.

If the alkali cellulose-caustic soda solution slurry is fed to the above-described separating device under constant composition and temperature conditions, the slurry press rolls will operate to deliver a constant volume of alkali cellulose at a constant press weight ratio. Since there will be unavoidable variations in the chemical and physical characteristics of the slurry, particularly in the swelling characteristics of the alkali cellulose, provision must be made for compensation of such variations so that the slurry press rolls will nevertheless operate to deliver a substantially constant volume of alkali cellulose having a substantially constant press weight ratio.

It is the primary object of this invention to provide a process and apparatus for the continuous production of alkali cellulose having a substantially constant press weight ratio.

It is another object of this invention to provide a process and apparatus for the continuous production of alkali cellulose having a substantially constant press weight ratio from an alkali cellulose-caustic soda solution slurry which is subject to unavoidable variations in the chemical and physical characteristics thereof.

Other objects of the invention will appear hereinafter.

The objects of this invention are accomplished, in general, by continuously feeding cellulose having different swelling characteristics into a slurry-forming receptacle, continuously adding sodium hydroxide solution to maintain a substantially constant volume of slurry and slurry forming mass in said receptacle, continuously passing, at a substantially constant pressure, a portion of the resulting alkali cellulose-caustic soda solution slurry from said receptacle to means for continuously removing the alkali cellulose from said slurry at a constant volume rate of alkali cellulose, and varying the temperature of said portion of slurry to compensate for differences in the swelling characteristics of the cellulose.

The following written description of the invention and apparatus involved in its operation will be more easily understood when taken in conjunction with the accompanying drawing wherein the single figure is a diagrammatic side elevational view, with parts shown in section, of one modification of apparatus suitable for use in carrying out the invention. I

Referring tothe drawing, numeral i0 designates a slurry forming receptacle which is preferably provided with a temperature controlling acket 8 and jacket inlet and outlet means H and i3, respectively. The Jacket inlet means ii may be provided with a throttling valve l5 controlled by a thermostatic temperature control it which is actuated by the temperature change within receptacle III. The receptacle I0 is provided with cellulose feed inlet l2 and. sodium hydroxide solution inlet means -9. The inlet means 8 is connected to a constant level supply means it comprising a supply duct I! for the sodium hydroxide solution and an overflow conduit i8. By this means suflicient solution will flow into receptacle III to maintain the slurry therein at a constant level. The receptacle I0 is provided with a draft tube IS in which is positioned a rotatable shaft 20. The shaft 20 which may be rutated by means of motor 2|, contains a plurality of stirring and cellulose disintegrating blades 22. The blades 22 cooperate with bars and blades on the internal periphery of the draft tube ll to tear and disintegrate the cellulose material and force the resulting slurry of alkali cellulose through the bottom of the draft tube IS. A portion of the slurry will pass upwardly between the tube 49 and receptacle ill to be again forced down through the draft tube. Another portion of the slurry will be drawn through slurry outlet 26 by means of centrifugal pump 21. An agitating device 25, may. if necessary, be positioned between outlet 28 and pump 21. The agitating device 25 will disintegrate any remaining pieces of cellulose substantlally into its fibrous state and thoroughly mix the same into the slurry. If desired, the device 25 may be provided with a screen, having for example, a inch mesh, to insure the formation of a substantially fibrous slurry mass. r

The pump 21 passes the slurry into conduit. 28 through which a minor portion is passed to a slurry separating tank 40. A heating jacket 23 is positioned about the section of conduit 23 which leads to separating tank 40. Another minor part of the slurry is passedfrom conduit 23 to branch conduit 3| through which it is passed to a pressure actuated viscosimeter 30. and from the viscosimeter 30, by means of conduit 32, back into receptacle Hi. The major portion of the slurry passing into conduit 23 is circulated back into receptacle Ill. A nozzle 33 is positioned on the end of conduit 23 which projects into receptacle it. This nozzle functions to pass the slurry into the receptacle with considerable force, and thereby forcing cellulose sheets 44 which may pass under nozzle 33 into the draft tube l3 to be disintegrated therein. Valve 35 in conduit 28 is preferably a diaphragm motor type valve which is controlled and throttled by means of pressure controller 34 to maintain a constant pressure in conduit 28 on the discharge side of pump 21. The pressure operated viscosimeter 30 which measures the rela tive viscosity of the slurry may be a pressure controller which operates to open or close valve 31 to admit steam into heating jacket 29 when the viscosity of the slurry exceeds a given value. Conduit 28 connects with the bottom of slurry separating container 30. The container is provided with slurry separating rolls 4i and 42 which are adapted to press the slurry forced in contact therewith to separate the solid alkali cellulose from the sodium hydroxide solution. The slurry separating container 40 is preferably constructed in the manner described in Patent No. 2.308.031 of W. R. Schmitz, Jr. The slurry press rolls are sealed in the container 40 to permit the slurry in the container to be maintained under pressure.

The slurry press rolls when rotating at a given peripheral speed will pass a substantially conby hand or by a feeding apparatus of the type described in Schmitz Patent No. 2.253.140. Such feeding of the cellulose is. however, always sub- .lect to slight fluctuations in feeding rate, due

(ill

to the'varlatlons in alpha cellulose content as brought about by variations in the moisture and ash contents of the pulp, or by unavoidable slight variation in feedingrate of the sheets. Aqueous sodium hydroxide solution is passed into receptacle In by means of a constant level supply device I 5. By means of such a device suilicient sodium hydroxide is constantly added to the slurry-forming receptacle to maintain the level of the slurry in the latter substantially constant. As the cell ulose is fed into the receptacle. it is guided. into the draft tube it in which the cellulose will be drawn by means of comminuting blades 22, which are rotated by means of motor 2| and shaft 2.. The cellulose will be torn and disintegrated and thoroughly mixed with sodium hydroxide solution to form a fibrous slurry mass of alkali cellulose. A portion of the slurry will be passed from the bottom of the draft tube l9 upwards between the draft tube l9 and receptacle Hi to again flow through the draft tube. Another portion of the slurry will be drawn from the outlet 26 of the receptacle by means of centrifugal pump 21. A portion of this slurry is forced through conduit 28 into the slurry separating device 40. Between pump 21 and device 40, the conduit 28 is provided with a heating jacket-29. Another portion of the slurry passes by means of conduit 31 to the pressure actuated viscosity measuring device 30 and thence back into receptacle 10 by means of conduit 32. The major portion of the slurry passes through valve 35 into receptacle 10 from nozzle outlet 33. Preferably a-heating or cooling fluid is passed through jacket 8 to maintain the temperature of the slurry constant. The quantity of fluid passed through the jacket is automatically regulated by throttling valve l5 and thermostatic control ll.

Throttling valve 35 in conduit 28 is controlled by pressure regulating controller 34 to regulate the flow of slurry to maintain the pressure in conduit 28 constant, for example, 10-20 lbs. per sq. in. gauge. The section of conduit 28 between pump 21 and device 40 is provided with a heating jacket .29. Steam, or other fluid. may be passed through jacket 29 to regulate the temperature of the slurry passing to device 40. Valve 31, which regulates the amount of steam admitted into the heating jacket is controlled by the pressure actuated vis cosimeter 30 which operates to open andv close valve 31 in accordance with increase or decrease in pressure in conduit 3|. Increase or decrease in pressure in conduit 3| is brought about by increase or decrease in viscosity of the slurry; therefore the valve 31 is in effect actuated by changes in viscosity of the slurry.

If the cellulose feeding rate is slightly increased. or if the cellulose momentarily being fed swells to a greater degree, the proportional amount of solid material in the slurry is slightly increased and therefore the viscosity of the slurry is slightly increased. Although the pressure in conduit 23 is maintained constant regardless of changes in viscosity of the slurry, the pressure will vary at the pressure actuated viscoslmeter 3II by such changes in viscosity due to changes in frictional drag throughout the length of conduit 3|. It is well known that in a given length of tube (conduit 3|) a viscosity variation from a given level is accompanied by an increase or decrease of pressure due to wall friction in the tube under the condition of a constant pressure on the inlet side, said variation being one in which the pressure increases with a drop in viscosity of the medium or decreases with a rise in viscosity of the slurry medium. It is further known that the viscosity of the above-described slurry is in pro ortion to the alkali cellulose content of the slurry, all other things being equal. Therefore, a pressure drop at viscosimeter 38 is an indication of an increase in alkali cellulose (solid) cnnten of the system.

The slurry is passed into the container In under a constant amount of pressure preferably between 10 and20 lbs. per sq. in. The slurry in the container M will be forced against rotating slurry press rolls ll and #2, which, upon rotation will press solid alkali cellulose from the liquid sodium hydroxide. The slurry press rolls 4-! and 42 are sealed in container 40 to permit the slurry contained therein to be maintained under the above mentioned pressure. The slurry press rolls are substantially non-yielding relative to each other and therefore pass a substantially constant uniform volume of alkali-cellulose per unit of time regardless of the fluctuating content of the alkali-cellulose in the slurry. As the alkali cellulose content (solid component) of the slurry is slightly increased the mats 45 of alkali cellulose deposited on the surfaces of rolls 4! and 42 are increased in thickness. The greater thickness of the mats t6 decreasesthe flow or discharge of'fluid int the inside of the rolls. The decrease in flow through the rolls transmits itself into a lower rate of flow through conduit 28. The rate at which the slurry is withdrawn from tank i0, is thereby decreased and since a constant incoming rate of cellulose is maintained, the concentration of alkali cellulose in the slurry increases. The increase in the alkali cellulose (solid) content produces a drop in pressure at the pressure actuated viscosimeter 30 (due to the increase in viscosity), which in turn causes a greater amount of steam to be admitted into heating jacket 29. The temperature of the slurry is then increased as it is passed into container it. The increased temperature of the slurry acts to decrease the swelling of the pulp which in turn makes a thinner mat on rolls 4i and 42. The diminishing thickness of the mats permits a greater flow of fluid from the system. which through the above sequence of operations acts to return the system to a normal operation.

The significance of above sequence of steps will be more apparent when it is understood that the slurry separating rolls act to deliver a constant volume of alkali cellulose. However, the percentage of solid alkali cellulose contained in this stated volume of alkali cellulose is proportional to the degree to which the alkali cellulose is swollen, the pressure in the container 60 pressing it against the roll surface, and the peripheral speed of roller 4i and 42, all other things being equal. Therefore, under conditions of constant peripheral speed and pressure. the solid alkali cellulose content of the discharged alkali cellulose mass is proportional to the degree to which the alkali cellulose is swollen. This means that the rate of discharge of solid alkali cellulose is then proportional to the swelling property of the same. It is known that for a given alkali cellulose the property swelling'between the ranges of 15 C. and C. is inversely proportional to the temperature. The rate of discharge of solid alkali cellulose is therefore directly proportional to the slurry temperature. It is therefore apparent that a variation in swelling of the alkali cellulose due to a change in the swelling characteristics of the cellulose being fed would act through the above cited sequence of eventsto regulate the temperature of the slurry to return the alkali cellulose swelling to a given level and maintain a given constant rate of production of uniform alkali cellulose at this level.

The constant level supply means for feeding sodium hydroxide solution to receptacle l0 may be a conventional float-controlled constant level supply device, or any other conventional means for feeding sodium hydroxide solution to maintain a constant level of the slurry in the slurry-forming receptacle. Many other similar changes and modifications can be made in the details of the a saasoe specifically illustrated and described apparatus.

Operation of the process and apparatus abovedescribed often involves the deposition of cal cium carbonate and other insoluble substances on the slurry press rolls M and 42. Such a deposit frequently reduces'the size of the perforations in these rolls. The process of the present invention will compensate for such reduction in size of the perforations by increasing the flow of heating fluid through jacket 29, thereby reducing the degree of swelling of the alkali cellulose.

In accordance with the present invention, alkali cellulose of uniform composition and containing a uniform percentage of sodium hydroxide solution can be produced in a continuous manner despite unavoidable fluctuations in the swelling characteristics of the cellulose and in the feeding rate of the cellulose to receptacle l0. By the process of the present invention, it is possible to use cellulose of somewhat varying characteristics to produce alkali cellulose of substantially uniform characteristics. 7

Since it is obvious that many changes and modifications can be made in the details above described, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. In a process for the continuous production of alkali cellulose of uniform composition, the steps comprising continuously forming an alkali cellulose-sodium hydroxide solution slurry containing alkali cellulose having differing swelling characteristics, continuously separating the alkali cellulose component from the sodium hydroxide solution component of said slurry and varying the temperature of the slurry between the slurry formation and the slurry separation to change the degree of swelling of the alkali cellulose in inverse proportion to the different swelling characteristics of alkali cellulose contained in the slurry.

2. In a process for the continuous production of alkali cellulose of uniform composition, the steps comprising continuously feeding cellulose having differing swelling characteristics into a slurry-forming receptacle, continuously feeding sumcient sodium hydroxide solution to said receptacle to maintain the level of mass therein substantially constant, agitating said mass to produce a slurry in which the alkali cellulose-component will have varying swelling characteristics slurry-forming receptacle, continuously feeding sufllcient sodium hydroxide solution to said receptacle to maintain the level of mass therein substantially constant, agitating said mass to produce a slurry having varying viscosity corresponding to the differing swelling characteristics of the cellulose, continuously passing, at a substantially constant pressure, a portion of the resulting alkali cellulose-caustic soda solution slurry from said receptacle and continuously separating the alkali cellulose from the caustic soda solution at a constant volume rate of alkali cellulose, and varying the temperature of the slurry cient sodium hydroxide solution to said receptacle to maintain the level of mass therein substantially' constant, agitating said mass to produce a slurry in which the alkali cellulose component will have varying swelling characteristics corresponding to the differing swelling characteristics of the cellulose, continuously passing, at a substantially constant pressure, a portion of the resulting alkali cellulose-caustic soda solution slurry from said receptacle and continuously separating the alkali cellulose from the caustic soda solution at a constant volume, rate of alkali cellulose, and varying the temperaturev of the slurry delivered to the separating zone to change the degree of swelling of the alkali cellulose'therein and thereby compensate for variations in the alkali cellulose content of the slurry.

5. In a process for the continuous production of alkali cellulose of uniform composition the steps comprising continuously passing through a conduit an alkali cellulose-sodium hydroxide solution slurry, containing alkali cellulose having differing swelling characteristics at a substantially constant pressure and continuously separating the alkali cellulose from the sodium hydroxide solution at a constant volume rate of alkali cellulose, and controlling the volume flow of said slurry delivered to the separating zone by regulating the temperature thereof.

6;. In a process for the continuous production of alkali cellulose of uniform composition, the steps comprisingcontinuously passing through a conduit an alkali cellulose-sodium hydroxide solution slurry, containing alkali cellulose having diilering swelling characteristics, at a substantially constant pressure and continuously separating the alkali cellulose from the sodium hydroxide solution at a constant volume rate of' alkali cellulose, and varying the temperature of the slurry delivered to the separating zone to compensate for variations in the swelling characteristics oi the alkali cellulose content of the slurry.

7. In an apparatus for the continuous production of alkali cellulose 01 uniiorm composition, a slurry-forming receptacle, a device for separating the solid phase from the liquid phase of said slurry,.means for passing slurry from said receptacle to said separating device, means for heating said slurry passing to said separating device, and means, operated by changes in viscosity of the slurry, for actuating said heating device.

8. In an apparatus, for the continuous production'oi alkali cellulose of uniform composition. a slurry-forming receptacle, a device for separating the solid phase from the liquid phase of said slurry, means for passing slurry from said receptacle to said separating device at a constant pressure, means for supplying heat to said slurry passing to said separating device, and means, operated by changes in viscosity of the slurry, for actuating said heat supplying means.

9. In an apparatus for the continuous production of alkali cellulose of uniform composition, a slurry-forming receptacle, a cellulose feed inlet in said receptacle; 9. constant-level liquid -suDPlY means connected to said receptacle, a

slurry outlet in said receptacle, a device for separating the solid phase from the liquid phase of said slurry, means for passing a slurry from said outlet to said separating means at a constant pressure, means for supplyingheat to slurry passing to said separating means, and means, operated by changes in the swelling characteristics of the alkali cellulose content of the slurry, ior actuating said heat supplying means.

10. In an apparatus for the continuous production of alkali cellulose of uniform composition, a slurry-forming receptacle, a device for separating the solid phase from the liquid phase of said slurry, means for passing slurry from said receptacle to said separating device under pr essure, means for maintaining constant said pressure, means for supplying heat to slurry passing from said receptacle to said separating device. and means, operated by changes in viscosity of the slurry, to actuate said heat supplying means.

WILIJAM RICHARD SCHMITZ, J3, 

